Sesshoumaru. (I LOVE him he's another Vergil to me ^^.) I somewhat agree with you Zato. Inuyasha and Sesshoumaru always have some kind of arguement when they have to help each other. Vergil and Dante didn't think twice about helping one another. It was almost instinct to them.
As for Inuyasha, let me go off topic for a sec. It seems as of late in the manga, Inuyasha is starting to love Sesshoumaru. (Not in that way!) At least he seems to care about him quite a bit. I can't say the same for Sesshoumaru, but anyways...
Back on topic: Morrigan, I respect your opinion, but I disagree a little bit. You see, I too have a sister that I can't stand. Now I don't hate her because she's my sister, but even though we sharded a childhood together, she's always treated me like crap and never been a real sister to me. Thats why sharing a childhood doesn't help in that department.
The twins however seemed happy together, allow me to assume that from that flashback in DMC1 they got along and where happy, laughing together and stuff. Thats why it would probably be alittle more easier for them, because they actually enjoyed their childhood together.